As being self-emission, EL devices have high self- distinguishability. In addition, they have high impact resistance as being completely solid devices. Therefore, the use of EL devices in various displays as light source is being widely noticed.
EL devices are grouped into inorganic EL devices in which are used inorganic compounds as light-emitting materials, and organic EL devices in which are used light-emitting organic compounds. Of those, organic EL devices have been being much studied and expected as practical light emitters in the coming generations, since they require a greatly reduced driving-voltage and they can be easily small-sized.
In the circumstances, the most important theme in practical studies of organic EL devices is to establish the technique of preventing the attenuation of the luminance of the devices in long-term driving and to provide practicable organic EL devices.
In this respect, it is known that the purity of organic compounds to be used for producing organic EL devices has a great influence on the attenuation of the luminous efficiency and the luminance of the devices produced, for example, as in "Monthly DISPLAY, September, p. 15, 1995", and "OYOBUTURI, Vol. 66, No. 2, pp. 114-115, 1997". However, the influences of the structures and the properties of organic compounds to be used for producing organic EL devices on the properties of the devices produced are not as yet clarified, and no method is known capable of quantitatively determine the influences in question.
Therefore, the details of the reasons why long-term use of organic EL devices brings about the attenuation of the luminance of the devices are not as yet known at present.